Composite sulfur electrode container and method of manufacture

ABSTRACT

A composite sulfur electrode container is described which comprises an outer metallic casing portion readily corroded by liquid sulfur and polysulfides, a metallic foil portion substantially corrosion resistant to liquid sulfur and polysulfides bonded to the inner surface of the outer metallic casing portion, and a layer portion of chromium bonded to the opposite surface of the foil portion, the chromium layer portion containing in excess of sixty weight percent chromium. A method is described for making a composite sulfur electrode container.

This invention relates to a composite sulfur electrode container and toa method for forming such a container and, more particularly, to such acomposite container including an outer metallic casing portion, ametallic foil portion, and a chromium layer portion bonded to its innersurface and to a method of forming such a container.

Sodium-sulfur cells, which operate at elevated temperatures, are knownin the prior art as, for example, described in U.S. Pat. No. 3,946,751issued Feb. 18, 1975, under the title "Cell Casing with a HermeticMechanical Seal and a Hermetically Sealed Sodium-Sulfur Cell".

In U.S. Pat. No. 3,959,013, there is described a cathode cell casingportion, a cell casing and a hermetically sealed sodium-sulfur cell. Acathode cell casing portion is formed from a metal selected fromaluminum, steel or iron-nickel-cobalt alloys. A corrosion resistant andelectronically conductive layer adheres to the inner surface of thecontainer, which layer is selected from the class consisting ofmolybdenum and graphite. One suitable method of applying and adheringthe molybdenum layer to the inner surface of the cell casing portion isto plasma spray such surfaces with a thin layer of molybdenum. Theabove-identified patents are assigned to the General Electric Company.

In U.S. Pat. No. 3,140,006, there is described a pressure vessel forcontaining hydrogen or mixtures thereof. In column one, lines 24-30,there is pointed out that vessels for containing hydrogen are knownwhich have a plurality of layers or sections wherein only the innerlayer or layers are resistant to hydrogen and the outer section orsections are of carbon steel.

The patent describes an improved construction for a pressure vesselhaving a suitable opening for admitting a fluid which comprises at leasttwo sections which are not metallurgically bonded, either directly orindirectly, and which have a gas-flow passage therebetween. The innersection is or contains a layer of material which is resistant todeterioration by gas, such as hydrogen absorbed in the metal. The outersection has weep holes inter-connecting the inner surface thereof withthe outside of the vessel. The outer section may then be constructed ofcarbon steel. This description is set forth in column two, lines 6-15and in column three, lines 56-59. In column three, lines 43-47, it ispointed out that the shell contains an inner section consisting of twolayers which may be bonded together as by rolling the layers ofdifferent metals to form a unitary sheet. The bonding may also beaccomplished by welds.

As opposed to the above patents, the present application describes andclaims a structure which has an outer metallic casing portion, ametallic foil portion bonded to the inner surface of the casing portion,and a chromium layer portion bonded to the opposite surface of the foilportion. The present composite sulfur electrode container does not haveand would not function with weep holes in the outer casing as describedin U.S. Pat. No. 3,140,006.

In U.S. Pat. application Ser. No. 837,381, filed Sept. 28, 1977, nowU.S. Pat. No. 4,131,226 under the title "Sulfur Electrode ContainerConstruction and Method of Manufacture", there is described a pluralityof sulfur electrode container constructions characterized by mild steeland discrete anti-corrosive liners disposed within the containers andmethods of manufacturing each. A discrete liner of a materialsubstantially non-corrodible by liquid sulfur is disposed within acontainer in substantially contiguous relation with respect to the innerwall of the container for sealing the same from the liquid sulfurtherein. A flexible anti-corrosion foil such as 347 stainless steel isrolled as a cladding into the shape of a cylindrical liner so that itfits snugly into the open ended rigid container. The edge margins of thefoil are disposed in slightly overlapping relation. The liner can bespot welded to the container along its overlapping edge margins.

In U.S. Patent application Ser. No. 889,046, filed Mar. 22, 1978, underthe title "A Sodium-Sulfur Cell Component Protected by a High ChromiumAlloy and Method of Forming", there is described an electricallyconductive corrosion resistant component for a sodium-sulfur cell whichincludes a ferrocarbon substrate and a chromium-iron-carbon alloysurface layer bonded to the substrate and containing in excess of 60weight percent chromium. The alloy surface layer is described further ashaving an outer layer of contiguous outer and inner chromium-iron-carbonduplex layers, in which the inner layer has a chromium content less thanabout 50 weight percent and which layer is in direct contact with thesubstrate.

A method is also described for forming a corrosion resistant layer on aferrocarbon substrate which includes pack chromizing the substrate bytotally submerging it in a solid pack of chromium source and a halideactivator at an elevated temperature to generate chromium halide atsufficient vapor pressure to diffuse the chromium into the substrate toform a chromium-iron-carbon alloy surface layer bonded to the substratecontaining in excess of about 60 weight percent chromium.

The above-described U.S. applications are assigned to the Electric PowerResearch Institute, Inc., subject to the reservation of license rightsto the General Electric Company. Our present invention is directed toproviding an improved composite sulfur electrode container and animproved method of manufacturing such a container over theabove-identified patents and patent applications in that there isutilized an outer metallic casing portion, a metallic foil portionbonded to the inner surface of the casing portion, and a chromium layerportion bonded to the opposite surface of the foil portion.

The primary objects of our invention are to provide an improvedcomposite sulfur electrode container and an improved method ofmanufacturing such a container wherein a self-sealing effect is producedby the foil and layer portions thereby minimizing creeping of thecorrosive sulfur or vapor between the foil and layer portions and theouter casing portion of the container.

In accordance with one aspect of our invention, a composite sulfurelectrode container includes a metallic outer casing portion, a metallicfoil portion, and a chromium layer portion bonded to its inner surface.The foil portion and the chromium layer portion are substantiallycorrosion resistant to liquid sulfur and polysulfides, while the outercasing portion is readily corroded by liquid sulfur and polysulfides.

These and various other objects, features and advantages of theinvention will be better understood from the following description takenin connection with the accompanying drawing in which:

FIG. 1 is a sectional view of a sodium-sulfur cell with a compositesulfur electrode container made in accordance with our invention;

FIG. 2 is a sectional view through the composite sulfur electrodecontainer of FIG. 1 taken on line 2--2 thereof;

FIG. 3 is an enlarged sectional view of a portion of the compositesulfur electrode container shown in FIG. 1; and

FIG. 4 is a schematic sectional view of apparatus for forming thecomposite sulfur electrode container of our invention.

In FIG. 1 of the drawing, there is shown a sodium-sulfur cell 10 of thetype described in above-mentioned U.S. Pat. No. 3,946,751 with theimproved composite sulfur electrode container of the present invention.Cell 10 has a ceramic ring 11, an inner casing of a solid sodiumion-conductive material 12 with one open end 13, and a glass seal 14sealing a portion of the outer wall 15 of inner casing 12 adjacent itsopen end 13 within and to the ceramic ring 11. An improved compositesulfur electrode container 16 has opposite open ends 17 and 18. An outermetallic casing portion 19 readily corroded by liquid sulfur andpolysulfides has a flange 20 at open end 17 of container 16. Container16 surrounds inner casing 12 and is spaced therefrom.

A single metallic foil portion 21 of 347 stainless steel which issubstantially corrosion resistant to liquid sulfur and polysulfides, isbonded to the inner surface of outer casing portion 19. A chromium layerportion 22 is bonded to the opposite surface of foil portion 21. Thelayer portion is substantially corrosion resistant to liquid sulfur andpolysulfides. A metallic end cap 23 with a foil portion 24 and chromiumlayer portion 25 bonded to its inner surface is shown fitted and sealedas by welding at 26 within and adjacent open end 18 of container 16. Asodium container 27 has opposite open ends 28 and 29 and a flange 30 atopen end 28. Sodium container 27 extends in an opposite direction tocomposite sulfur electrode container 16. A metallic end cap 31 foropposite open end 29 of sodium container 27, has a fill opening 32 inend cap 29, and a fill tube 33 affixed to end cap 29 and incommunication with fill opening 32.

Sulfur and sodium metallic containers 16 and 27 are shown joined toceramic ring 11 by a hermetic mechanical seal shown generally at 34thereby forming a continuous container. Seal 34 comprises a pair ofretainer rings 35, each of which is positioned between ceramic ring 11and an adjacent flange 20 or 30 of containers 16 and 27, respectively. Apair of "C" shaped sealing rings 36 are positioned similarly to rings 35but surround exteriorly and are spaced from rings 35. The open portionof each "C" shaped sealing ring faces outwardly. A retaining collar 37is positioned around each container and adjacent to the opposite surfaceof the casing flange. Each collar 37 has at least a pair of andpreferably a plurality of apertures 38 therethrough. The collars arepositioned so that the respective apertures 38 are aligned. Threadedfasteners 39, each passing through a pair of associated apertures, areemployed to tighten the mechanical seal to produce a hermetic mechanicalseal 34. Electrical insulation 40 in the form of fiberglass tape isshown wound around the exterior surface of containers 16 and 27 adjacentthe respective flanges 20 and 30 to prevent short-circuiting of the cellby seal 34. Electrical insulation 41 in the form of an iorganic fibercloth ring is positioned between each flange 20 and 30 and the surfaceof each associated collar 33 to prevent short-circuiting of the cell byseal 34. While the opposite surfaces of ceramic ring 11 are smooth toinsure a good seal, there is shown also a preferred ring 42 of aluminumfoil between the opposite surface of retainer ring 35 and sealing ring36 and the associated surface of ceramic ring 11 to provide a smoothersurface. Hermetic mechanical seal 34 is shown in its tightened orhermetic position. A negative electrode 43 of sodium metal is positionedpreferably within inner casing 12 and partially within sodium container27. A positive electrode 44 of a sulfur-carbon plug is positionedpreferably within container 16 and is in contact with outer wall 15 ofinner vessel 12 and with chromium layer portion 22. A void volume isprovided between ceramic ring 11 and the upper portion of positiveelectrode 44 to provide space for reactant during operation of the cell.Fill tube 33 is shown closed in any suitable manner such as by a weld45. End cap 31 is affixed to container 27 as by welding at 46. Theresulting structure is a hermetically sealed sodium-sulfur cell.

In FIG. 2 of the drawing, there is shown a sectional view through theimproved composite sulfur electrode container of FIG. 1 taken on line2--2 thereof. The same numerals are used in the descriptive of FIG. 2 aswere employed in the description of FIG. 1. Composite sulfur electrodecontainer 16 surrounds inner casing 12 and is spaced therefrom. Metallicfoil portion 21 is bonded to the inner surface of outer metallic casingportion 19. Chromium layer portion 22 is bonded to the opposite surfaceof foil portion 21. Negative electrode 43 of sodium metal is positionedwithin inner casing 12. Positive electrode 44 of a sulfur-carbon plug ispositioned within container 16 and in contact with outer wall 15 ofinner vessel 12.

In FIG. 3 of the drawing, there is shown an enlarged sectional view of aportion of composite sulfur electrode container 16 which is shown inFIG. 1. Composite sulfur electrode container 16 has an outer metalliccasing portion 19 readily corrodible by liquid sulfur and polysulfides.Metallic foil porton 21, which is substantially corrosion resistant toliquid sulfur and polysulfides, is bonded to the inner surface of outermetallic casing portion 19. A chromium layer portion 22, which issubstantially corrosion resistant to liquid sulfur and polysulfides isbonded to the opposite surface of foil portion 21.

In FIG. 4 of the drawing, there is shown a schematic sectional view ofan apparatus for forming the composite sulfur electrode container of ourinvention. Apparatus 50 has an enclosure 51 such as a box type furnace.An inert gas or hydrogen is directed through inlet 52 into the interiorof enclosure 51 and removed through an outlet 53 from enclosure 51. Astainless steel retort 54 is positioned within enclosure 51. Retort 54is shown as having a bottom wall 55 and a side wall 56. The upper end ofretort 54 has an open end as shown at 57. A chromizing pack 58substantially fills retort 54. A suitable chromizing pack 58 includes10-60 weight percent chromium powder as the chromium source, about 2.8percent ammonium chloride and 50-80 weight percent alpha-alumina powderas a filler. To avoid sintering of the chromium powder and for economy,it is preferable to employ below 50 weight percent chromium powder. Ifdesired, a ferrochromium alloy may be substituted for chromium as thechromium source. A sulfur electrode container 59 is positioned in aninverted position and totally submerged in chromizing pack 58. A sulfurelectrode container comprises an outer metallic casing portion 60 havingan open end 61 and a flange 62 at the open end. A single metallicportion 63 of 347 stainless steel is positioned within outer casingportion 60 and has its one surface in contiguous relation to the innersurface of outer casing portion 60. A metallic end cap 64 is positionedwithin the opposite or upper end of outer casing 60 and sealed to outercasing portion 60 and metallic foil portion 63 by means of a weld 65.End cap 64 has a single metallic foil portion 66 disposed on its innersurface. Enclosure 51 is heated to a suitable temperature from about850°-1200° C. to produce a chromium layer on the inner surface of foilportion 63 and on the inner surface of foil portion 66. Additionally,the apparatus diffuses chromium into foil portions 63 and 66, and bondsfoil portions 63 and 66 to the inner surface of outer metallic casing 60and end cap 64, respectively.

We found that we could form an improved composite sulfur electrodecontainer which includes an outer metallic casing portion readilycorroded by liquid sulfur and polysulfides, such as low carbon steel.Another suitable outer metal casing portion material is nickel. Thecontainer has opposite open ends. It may be desirable, depending on thetype of composite sulfur electrode container and on the method ofjoining such container to form a sodium-sulfur battery, to provide anoutwardly or inwardly extending flange at one end of outer casingportion. As it is shown in both FIGS. 1 and 2 of the drawing, anoutwardly extending flange is employed. Such a flange configuration isused in view of the hermetic mechanical seal employed in thesodium-sulfur battery in which it is used. Since other sealing methodsare available for affixing the composite sulfur electrode container as aportion of the sodium-sulfur cell, such an outwardly extending flangemight not be required. However, we will describe an embodiment of thecomposite sulfur electrode container of our invention which can beemployed in the cell shown in the Figures of the drawing and describedabove. A metallic foil portion is provided, which is substantiallycorrosion resistant to liquid sulfur and polysulfides. Suitablematerials for the foil include various stainless steels such as 347stainless steel, and nickel-chromium alloys. The foil portion is rolledinto a cylindrical shape and is positioned within the outer metalliccasing whereby the outer surface of the foil portion is in contiguousrelation with the inner surface of the outer metallic casing. The edgemargins of the foil portion are disposed in a slightly overlappingrelation. The foil portion is preferably coextensive with the innersurface of the outer metallic casing. If desired, one or more tack weldsmay be made to adhere initially the foil to the inner surface of theouter metallic casing.

We found that we could then position a metallic end cap with a foilportion on its inner surface within the open end of the containeropposite to the open end having a flange. The metallic end cap is thensealed as by welding within and adjacent the open end of the container.In another embodiment of the invention, the end cap is not positioned inthe open end of the container, nor sealed therein until a later step inthe formation of the container. We found that we could than chromize theinner surface of the foil portions adjacent the side wall and the endcap and also diffuse chromium into the foil portion. We found further,that the chromizing bonded the foil portion to the inner surface of theouter metallic casing and bonded the chromium layer to the oppositesurface of the foil portion. We found further, that the foil portion,after chromizing, would have some additional chromium diffused into thefoil during the process, while the chromium layer portion would containin excess of 60 weight percent chromium.

We found that we could chromize, by pack chromizing method. It will beappreciated that other chromizing processes such as chromizing bychemical vapor deposition from a separate vapor source can be employed.The pack chromizing method included positioning the above describedouter metallic casing with foil portion positioned therein and the endcap with foil portion on its inner surface welded thereto in an invertedposition within a chromizing pack in a stainless steel retort, whichretort is positioned within an enclosure. An inert gas, such as argon orhydrogen, is flowed into and out from the enclosure while the pack isheated to a suitable temperature in the range from 850° to 1150° C. Thepreferred temperature range is from 950° to 1100° C. At temperatures inexcess of 1150° C., it is possible that the chromium layer portion willbecome too thin for effective protection. Generally, the chromium layerportion should have a thickness two microns or thicker. The time periodshould be as short as possible from an economy standpoint. However, itis necessary that the time period be sufficient to provide a chromiumlayer portion having an effective thickness and containing in excess of60 weight percent chromium. We found that we could provide a suitablechromium layer portion in a period as short as one-half hour to periodsof, for example, three to four hours. The preferred time period is onehour. The chromium pack includes a chromium source and a halideactivator, preferably amonium chloride or bromide. During the chromizingprocess, chromium halide is generated whereby chromium is diffused intothe foil portion, forms a chromium layer portion on the foil portion,and bonds the foil portion to the inner surface of the outer metalliccasing. A suitable chromizing pack includes 10 to 60 weight percentchromium powder as the chromium source, about 2 weight percent amoniumchloride and the balance alpha-alumina powder as a filler. To avoidsintering of the chromium powder and for economy, it is preferable toemploy below 50 weight percent chromium powder. If desired, aferrochromium alloy may be substituted for chromium as the source.

We found that it is necessary to provide a heated vapor containingchromium from a chromium source at a sufficient vapor pressure todiffuse chromium into the foil portion, bond the foil portion to theinner surface of the outer metallic casing portion, and form, and bond achromium layer portion to the opposite surface of the foil portion. Thevapor pressure of chromium halide increases with temperature, and variesdepending upon the particular halide. A high concentration of thechromium halide adjacent to the inner surface of the outer metalliccasing portion with foil portion therein, appears to contribute to asignificant extent to the chromizing step. If a sufficient vaporpressure is not employed, the chromium layer portion containing inexcess of 60 weight percent chromium is not formed. Thus, the vaporpressure should be sufficient to form a chromium layer portion of athickness two microns or thicker.

Subsequent to the chromizing, the resulting device is a composite sulfurelectrode container made in accordance with our invention.

In the pack chromizing process, the assembly of the outer metalliccasing and the end cap with their respective foil portions, ispreferably inverted so that the end cap is near the upper portion of thechromium pack. As it is mentioned above, it may be desirable to provideand weld the end cap to the outer metallic casing and its foil portionsubsequent to the chromizing process. When this is accomplished, it isnecessary to position a sheet of material over the open upper end of theouter metallic casing or over the top of the pack to provide thechromizing step necessary to form our container.

A sodium-sulfur cell is then assembled as above described, employing theimproved composite sulfur electrode container of our invention. In theassembly of the composite sulfur electrode container in thesodium-sulfur cell, as shown in FIGS. 1 and 2 of the drawing, thesulfur-carbon plug is inserted within the container and in contact withits chromium layer portion from the opposite open end of the compositesulfur electrode container. The composite sulfur electrode container,including the sulfur-carbon plug, is then hermetically sealed by meansof its flange to the ceramic ring in the sodium-sulfur cell.

Examples of composite sulfur electrode containers and method ofmanufacturing such containers made in accordance with our invention, areset forth below:

EXAMPLE I

A composite sulfur electrode container was formed as above described andas is shown in FIGS. 1 and 2 of the drawing by providing an outermetallic casing portion of low carbon steel having opposite open ends,which container is readily corrodible by liquid sulfur and polysulfides.An outwardly extending flange is provided at one open end of the outercasing. A 2 mil. thick 347 stainless steel foil portion, which issubstantially corrosion resistant to liquid sulfur and polysulfides, wasformed into a cylinder with overlapping margins and positioned withinthe outer casing. The outer surface of the foil portion was insubstantially contiguous relation with respect to the inner surface ofthe outer casing. A metallic end cap of low carbon steel having a foilportion disposed on its inner surface was fitted within and adjacent theopen end of the casing portion opposite the open end with the flange sothat the edge of the end cap was in contact with the inner surface ofthe foil portion. The end cap was sealed by welding the cap to the foilportion and the outer casing portion to provide an assembly.

The assembly is positioned in a chromizing pack within a stainless steelretort in an inverted position so that the end cap or closed end of theassembly is in the upper part of the chromizing pack. The chromizingpack comprised 40 weight percent chromium, 2 weight percent amoniumchloride and the balance alpha-alumina powder. The retort was positionedwithin an enclosure into and from which hydrogen gas was flowed. Thechromizing was carried out at 1050° C. for one hour. The chromizingbonded the foil portion to the inner surface of the outer metalliccasing portion, diffused chromium into the foil portion, formed andbonded a chromium layer portion to the opposite surface of the foilportion, which chromium layer portion content was about 80 weightpercent chromium. The resulting device was a composite sulfur electrodecontainer made in accordance with our invention.

EXAMPLE II

The composite sulfur electrode container of Example I was employed in asodium-sulfur cell of the type described above and shown in FIGS. 1 and2 of the drawing. The cell had a ceramic ring of alpha alumina, an innercasing of solid sodium and beta alumina in tube form with one open end,and a glass seal sealing a portion of the outer wall of the inner casingadjacent its open end within and to the ceramic ring. A sodium containerof metal had opposite open ends and a flange at one open end. An end capwas welded to the open end of the sodium container opposite to the openend having a flange. A fill opening was provided in the end cap and afill tube affixed to the end cap and in communication with the fillopening.

A ring of aluminum foil was positioned on the upper surface of theceramic ring. A retainer ring is positioned on the upper surface of thealuminum foil and surrounded by "C" shaped sealing ring with its openingfacing outwardly. The flange of the sodium container is positioned onthe upper surface of both the retainer ring and the "C" shaped sealingring. Fiberglass tape was wound around the exterior surface of thesodium container adjacent its flange. An inorganic fiber cloth ring waspositioned around the upper surface of the flange of the sodiumcontainer.

The composite sulfur electrode container of Example I had asulfur-carbon plug inserted therein through its open end. The sulfurelectrode container, with its plug positioned therein surrounded thebeta-alumina tube, whereby the tube was fitted into the opening withinthe sulfur-carbon plug. As described above, the sulfur-carbon plug waspositioned on the opposite surface of the ceramic ring in the samemanner as was the sodium container. A retaining collar was positionedaround each container and adjacent to the opposite surface of the casingportion flange. Each collar had a plurality of apertures therethrough.The collars were positioned so that the respective apertures werealigned. A threaded fastener passed through each pair of associatedapertures. These fasteners were then tightened to provide a hermeticmechanical seal for the cell. The threaded fasteners were tightened to apressure of about 300 lbs. to provide a complete hermetic mechanicalseal.

EXAMPLE III

The cell of Example II was positioned in a furnace and heated to a celloperating temperature of 315° C. The cell was charged and dischargedover 100 cycles in a completely satisfactory manner. Subsequently, thecell was disassembled and the inner surface of the composite sulfurelectrode container was examined. The container showed only nominalamounts of corrosion products.

While other modifications of the invention and variations thereof whichmay be employed within the scope of the invention have not beendescribed, the invention is intended to include such as may be embracedwithin the following claims:

What we claim as new and desire to secure by Letters Patent of theUnited States is:
 1. A composite sulfur electrode container for use as aportion of a sodium-sulfur cell which comprises an outer metallic casingportion readily corroded by liquid sulfur and polysulfides, the outercasing having opposite open ends, a metallic foil portion substantiallycorrosion resistant to liquid sulfur and polysulfides bonded to theinner surface of the outer casing portion, a chromium layer portionbonded to the opposite surface of the foil portion, the chromium layerportion containing in excess of 60 weight percent chromium, a metallicend cap adapted to fit and to be sealed within and adjacent one open endof the container, and at least the inner surface of the end capsubstantially corrosion resistant to liquid sulfur and polysulfides. 2.A composite sulfur electrode container as in claim 1, in which the endcap is fitted and sealed within and adjacent one open end of thecontainer.
 3. A composite sulfur electrode container as in claim 1, inwhich the end cap is fitted and sealed within and adjacent one open endof the container, and the inner surface of the end cap consists of afoil portion bonded to the end cap and a chromium layer bonded to theopposite surface of the foil portion, the chromium layer containing inexcess of 60 weight percent chromium.